In a semiconductor device (for example, a hybrid IC) in which various types of semiconductors are mounted, air-tight sealing is carried out for the purpose of shutting off the semiconductors from the outside air. In a semiconductor device in which oscillating modules are mounted on the same circuit substrate, the provision of electromagnetic shielding characteristics are also required in addition to the air-tight sealing characteristics.
A metal case sealed by a resistance welding method (Japanese Utility Model Laid-open No. 33482/1990) is generally known as a package structure having both the air-tight sealing characteristics and electromagnetic shielding characteristics as mentioned above.
This metal case consists of a combination of an external lead-retaining stem and a cap. In this metal case, a semiconductor device formed by setting a semiconductor element on a circuit substrate and bonding the former to the latter with a gold wire is fixed to a stem, and a cap is put on the resultant product so as to seal the same.
A package structure produced by forming a second conductive layer on an intermediate layer provided on a circuit substrate, putting a flangeless cap on the resultant product, and soldering the end surface of the outer circumferential portion of the circuit substrate to electrically connect the second conductive layer and cap together and electro-magnetically shield the product has also been proposed recently (Japanese Utility Model Laid-open No. 13796/ 1990).
In the former package structure out of these prior art package structures, a metal case (stem and cap) in which the circuit substrate is housed is required, and this necessarily causes the manufacturing cost and the weight of the product to increase. Moreover, a dedicated sealing apparatus is indispensable for the production of this package structure. Therefore, it is very difficult for this package structure to meet a demand, which has been made in recent years, to reduce the weight, height, length and width thereof.
The latter package structure is formed by merely putting a cap on the circuit substrate, so that the dimensions and weight thereof can be reduced. However, since the cap is soldered to the end surface of the circuit substrate, it is necessary to separate the circuit substrate into parts and seal the parts separately, so that the operation efficiency becomes low. Therefore, this package structure is not suitable for the mass production of the devices of this kind.
In an electronic part having a plurality of semiconductor elements mounted on a single substrate, and a plurality of external electrodes, such as a hybrid IC, it is important to miniaturize a package and reduce the packaging area (improve the packaging density). Although various package structures have been developed, a leadless package structure is basically most advantageous in order to reduce the packaging area (refer, for example, to Japanese Patent Laid-open No. 84681/1978).
Various structures of an external electrode in a conventional leadless package have also been developed. These structures include, for example, a structure (Japanese Utility Model Laid-open No. 168141/1983) in which the circuit pattern provided on the packaging surface (bottom surface) of a ceramic substrate is used as it is as an external electrode, as well as a structure (Japanese Utility Model Laid-open No. 114941/1990) in which projections of regular solder are formed on a conductive pattern provided on the packaging surface of a ceramic substrate and used for forming an external electrode thereon. Such leadless parts are mounted on a packaging substrate (printed wiring board) and soldered to a circuit pattern by a reflow method or by a flow method, whereby the electrical and mechanical connection of the parts is carried out.
In these external electrode structures, the ceramic substrate and packaging substrate are liable to closely contact each other during a packaging operation, so that it is impossible to visually ascertain the electrical connection of a leadless part to a circuit pattern via solder. Even when the package structure is washed, flux remains thereon or solder balls (excess solder in the form of balls) are deposited thereon, and this would cause short-circuiting to occur. Moreover, since the ceramic substrate and packaging substrate have different thermal expansion coefficients and Young's moduli, flexure occurs easily in the connected portions of these substrates due to the stress occurring therein and attributable to the heat cycle, and due to the deformation thereof, such as a warp thereof.
A structure which is capable of partially solving these problems, and which uses hemispherical projections of conductive rubber as external electrodes, has also been proposed (Japanese Patent Laid-open No. 96751/1984). In this external electrode structure, the flexure of the ceramic substrate and packaging substrates can be prevented by the conductive rubber but solder cannot be used to connect a leadless part to the packaging substrate. Therefore, a leadless part is necessarily pressed against the packaging substrate by an auxiliary means or a thermocompression bonding method is necessarily employed. Moreover, it is necessary that the projections of conductive rubber be fixed one by one to predetermined portions of a ceramic substrate, so that the manday becomes high.